A linear stepping motor that is used to directly obtain a linear driving force can linearly moves a movable member by a predetermined pitch in accordance with input currents step by step. Therefore, the linear stepping motor inherently has a self positioning function, and positioning can be performed with an open-loop control system.
In the linear stepping motor, a support member for a movable member should be essentially provided in order to uniformly maintain a gap between the stator and the movable member. Examples of the support member include a mechanical support member which uses a linear bearing, a roller, or a linear guide, a pneumatic support member which applies pneumatic pressure into the gap between the stator and the movable member to keep the gap uniformly, and a magnetic levitation-type support member which supports the movable member without contact by controlling magnetic forces.
As a prior art to this invention, a double-sided linear stepping motor will be described referring to FIGS. 1 to 3.
FIG. 1 is a perspective view partially showing the double-sided linear stepping motor according to a prior art. FIG. 2 is a side sectional view showing a stator and movable members of the linear stepping motor shown in FIG. 1. The linear stepping motor shown in FIGS. 1 and 2 uses a mechanical support member.
FIG. 1 shows a state where a left yoke and a housing of a stator 40 are removed so that the structures of a stator 40, and upper and lower movable members 10 and 20 are exposed. As shown in FIGS. 1 and 2, the linear stepping motor includes an upper movable member 10 that has two cores 11 and 11′ protruding downward; a lower movable member 20 that has two cores 21 and 21′ protruding upward; two upper coils 30 and 30′ that are wound around the upper cores 11 and 11′ of the upper movable member 10, respectively; two lower coils 31 and 31′ that are wound around the lower cores 21 and 21′ of the lower movable member 20, respectively; and a flat plate-shaped stator 40 that is located between the upper movable member 10 and the lower movable member 20. The upper movable member 10 and the lower movable member 20 are connected to each other by a yoke 50, and move together with the stator 40 interposed between. A plurality of teeth 12 and 12′ are formed at lower ends of the upper cores 11 and 11′, respectively, to protrude downward at regular pitch. Symmetrically, a plurality of teeth 22 and 22′ are formed at upper ends of the lower cores 21 and 21′, respectively, to protrude upward at regular pitch, too. And many teeth 41 are symmetrically formed on the upper and lower surfaces of the stator 40 at regular pitch.
A guide 51 is extended from the yoke 50, and movably supported by many roller bearings 62 which are fitted in a guide rail 13 fixed to a housing 60.
FIG. 3 is a graph showing the phase of current applied to each coil shown in FIGS. 1 and 2. In FIG. 3, an A-phase current is applied to the first upper coil 30 and the first lower coil 31. A B-phase current is applied to the second upper coil 30′ and the second lower coil 31′. An electro-magnetic field generated by the currents has four modes in accordance with a phase difference between the A-phase current and the B-phase current. Electro-magnetic forces between the stator and the movable members are changed at a predetermined cycle depending on the modes of the electro-magnetic field, which leads to the linear movement of the movable members by a predetermined pitch. Referring to FIGS. 1 and 2, it is clear that the movable members moves by ¼ pitch due to a relative position between the teeth of the stator 40 and the movable members each time the input electro-magnetic field is switched in mode.
As will be apparent from FIG. 1, in order to keep constant air gap between the stator and the movable members, the guide 51, the guide rail 13, and a roller 62 inserted between them are essentially provided. But such a mechanical support member often causes friction and lubrication problems. Instead of mechanical support members, other support members such as air bearings or linear magnetic bearings may be used. However, the addition of such a non-contact support member causes a complex and bulky structure of the linear stepping motor system.